U.S. patent number 10,237,974 [Application Number 15/400,218] was granted by the patent office on 2019-03-19 for metal nanowire thin-films.
This patent grant is currently assigned to RAMOT AT TEL-AVIV UNIVERSITY LTD. ISRAELI COMPANY OF. The grantee listed for this patent is RAMOT AT TEL-AVIV UNIVERSITY LTD. Invention is credited to Daniel Azulai, Olga Krichevski, Gil Markovich.
![](/patent/grant/10237974/US10237974-20190319-D00000.png)
![](/patent/grant/10237974/US10237974-20190319-D00001.png)
![](/patent/grant/10237974/US10237974-20190319-D00002.png)
![](/patent/grant/10237974/US10237974-20190319-D00003.png)
![](/patent/grant/10237974/US10237974-20190319-D00004.png)
![](/patent/grant/10237974/US10237974-20190319-D00005.png)
![](/patent/grant/10237974/US10237974-20190319-D00006.png)
United States Patent |
10,237,974 |
Markovich , et al. |
March 19, 2019 |
Metal nanowire thin-films
Abstract
A conductive nanowire film having a high aspect-ratio metal is
described. The nanowire film is produced by inducing metal
reduction in a concentrated surfactant solution containing metal
precursor ions, a surfactant and a reducing agent. The metal
nanostructures demonstrate utility in a great variety of
applications.
Inventors: |
Markovich; Gil (Tel Aviv,
IL), Azulai; Daniel (Rehovot, IL),
Krichevski; Olga (Ariel, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
RAMOT AT TEL-AVIV UNIVERSITY LTD |
Tel-Aviv |
N/A |
IL |
|
|
Assignee: |
RAMOT AT TEL-AVIV UNIVERSITY LTD.
ISRAELI COMPANY OF (Tel Aviv, IL)
|
Family
ID: |
41650291 |
Appl.
No.: |
15/400,218 |
Filed: |
January 6, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170127515 A1 |
May 4, 2017 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
13061745 |
|
9574272 |
|
|
|
PCT/IL2009/000842 |
Sep 1, 2009 |
|
|
|
|
61190712 |
Sep 2, 2008 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01B
1/02 (20130101); H05K 1/0274 (20130101); H01L
29/786 (20130101); C23C 18/48 (20130101); H05K
3/105 (20130101); H05K 1/09 (20130101); C23C
18/34 (20130101); C23C 18/166 (20130101); C23C
18/44 (20130101); C23C 18/1676 (20130101); C23C
18/40 (20130101); C23C 18/143 (20190501); C23C
18/1667 (20130101); H05K 2203/1157 (20130101); H05K
2201/0257 (20130101); H05K 2203/013 (20130101); H05K
2201/10128 (20130101); H05K 2203/125 (20130101); Y10T
428/24917 (20150115); H05K 2201/0108 (20130101); H05K
2201/10121 (20130101); H05K 2201/026 (20130101); H05K
3/182 (20130101); H05K 2201/10166 (20130101) |
Current International
Class: |
B32B
3/00 (20060101); C23C 18/16 (20060101); H05K
1/02 (20060101); H05K 1/09 (20060101); C23C
18/14 (20060101); C23C 18/34 (20060101); C23C
18/40 (20060101); C23C 18/44 (20060101); C23C
18/48 (20060101); H05K 3/10 (20060101); H01B
1/02 (20060101); H01L 29/786 (20060101); H05K
3/18 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2363891 |
|
Sep 2011 |
|
EP |
|
2010026571 |
|
Mar 2010 |
|
WO |
|
2010130986 |
|
Nov 2010 |
|
WO |
|
Other References
Ahn et al.; "Transparent conductive grids via direct writing of
silver nanoparticie inks"; Nanoscale, vol. 3; 2011; pp. 2700-2702.
cited by applicant .
Azulai et al,; "On-Surface Formation of Metal Nanowire Transparent
Top Electrodes on CdSe Nanowire Array-Based Photoconductive
Devices"; ACS Publications, vol. 4, No. 6; 2012; pp. 3157-3162.
cited by applicant .
Azulai et al,; "Seed Concentration Control of Metal Nanowire
Diameter"; NANO Letters, vol. 12, No. 11; 2012 pp. 5552-5558. cited
by applicant .
Azulai et al.; "Transparent Metal Nanowire Thin Films Prepared in
Mesostructured Templates", Nano Letters, vol. 9, No. 12, Oct. 23,
2009; pp. 4246-4249. cited by applicant .
Belenkova et al.; "UV induced formation of transparent Au--Ag
nanowire mesh film for repairable OLED devices"; Journal of
Materials Chemistry, vol. 22; 2012; pp. 24042-24047. cited by
applicant .
De et al.; "Silver Nanowire Networks as Flexible, Transparent,
Conducting Films: Extremely High DC to Optical Conductivity
Ratios"; ACS Nano, vol. 3, No. 7; 2009; pp. 1767-1774. cited by
applicant .
Hellstrom et al.; "Polymer-Assisted Direct Deposition of Uniform
carbon nanotube bundle networks for high performance transparent
Electrodes"; ACS Nano, vol. 3, No. 6; 2009; pp. 1423-1430. cited by
applicant .
Hu et al.; "Scalable Coating and Properties of Transparent,
Flexible, Silver Nanowire Electrodes"; ACS Nano, vol. 4, No. 5;
2010; pp. 2955-2963--Abstract only. cited by applicant .
Hubert et al.; "Cetyltrimethylammoniurn Bromide Silver Bromide
Complex as the Capping Agent of Gold Nanrods"; Langmuir; vol. 28;
2008; pp. 9219-9222. cited by applicant .
Huo et al.; "Sub-Two Nanometer Single Crystal Au Nanowire"; Nano
Lett., vol. 8, No. 7; 2008; pp. 2041-2044--Abstract only. cited by
applicant .
International Search Report for Internation Application No.
PCT/IL2009/000842; International Filing Date Sep. 1, 2009; dated
Aug. 30, 2010; 2 pages. cited by applicant .
International Search Report for International Application No.
PCT/IL2013/050184; International Filing Date Feb. 28, 2013; dated
May 28, 2013; 10 pages. cited by applicant .
Jana et al.; "Liquid crystalline assemblies of ordered gold
nanorods"; J. Mater. Chem., vol. 12; 2002; pp. 2909-2912. cited by
applicant .
Jana et al.; "Wet Chemical Synthesis of High Aspect Ratio
Cylindrical Gold Nanorods"; J. Phys. Chem. B , vol. 105; 2001; pp.
4065-4067. cited by applicant .
Jung-Yong Lee, "Solution-Processed Metal Nanowire Mesh Transparent
Electrodes", Nano Letters, 8, 689-692 (2008). cited by applicant
.
Kang et al.; "Nanoimprinted Semitransparent Metal Electrodes and
Their Application in Organic Light-Emitting Diodes"; Advanced
Materials, vol. 19; 2007; 1391-1396. cited by applicant .
Kang et al.; "Organic Solar Cells Using Nanoimprinted Transparent
Metal Electrodes"; Adv. Mater., vol. 20; 2008; pp. 4408-4413. cited
by applicant .
Krichevski et al.; "Growth of Au/Ag nanowires in thin surfactant
solution films: an electron microscopy study"; J. Colloid Interface
Sci., vol. 314, 2007; pp. 304-309. cited by applicant .
Krichevski et al.; "Growth of Colloidal Gold Nanostars and
Nanowires induced by Palladium Doping"; Langmuir, vol. 23; 2007;
pp. 1496-1499. cited by applicant .
Krichevski O. et al., "Formation of Gold-Siiver Nanowires in Thin
Sufactant Solution Films", Langmuir, vol. 22, No. 3, 2006, 867-808.
cited by applicant .
Kumar et al,; "The Race to Replace Tin-Doped Indium Oxide: Which
Material Will Wiri?"; ACS Nano, vol. 4, No. 1; 2010; pp. 11-14.
cited by applicant .
Kuo et al.; "Synthesis of Branched Gold Nanocrystals by a Seeding
Growth Approach"; Langmuir, vol. 21, No. 5; 2005; pp. 2012-2016.
cited by applicant .
Layani et al.; "Flexible transparent conductive coatings by
combining self-assembly with sintering of silver nanoparticles
performed at room temperature"; Journal of Materials Chemistry,
vol. 21; 2011; pp. 15378-15382. cited by applicant .
Lee et al.; "Control of Current Saturation and Threshoid Voltage
Shift in Indium Oxide Nanowire Transistors with Femtosecond Laser
Annealing"; ACS Nano, vol. 5, No. 2; 2011; pp. 1095-1101--Abstract
only. cited by applicant .
Lee et al.; "Solution-Processed Metal Nanowire Mesh Transparent
Electrodes"; Nano Letters, vol. 8, No. 2; 2008; pp. 689-692. cited
by applicant .
Lu et al.; "Continuous formation of supported cubic and hexagonal
mesoporous films by sol-gel dip-coating"; Nature, vol. 389; Sep.
25, 1997; pp. 364-368. cited by applicant .
Lu et al.; "Ultrathin Gold Nanowires Can Be Obtained by Reducing
Polymeric Strands of Oleylarnin-Auel Complexes Formed via
Aurophilic Interaction"; J. Am. Chem. Soc., vol. 130, No. 28; 2008;
pp. 8900-8901. cited by applicant .
Lyons et al,; "High-Performance Transparent Conductors from
Networks of Gold Nanowires"; J. Phys. Chem, Lett., vol. 2; 2011;
pp. 3058-3062--Abstract only. cited by applicant .
Mortier, et al.; "Two-step synthesis of high aspect ration gold
nanorods"; Central European Journal of Chemistry, vol. 4, No. 1;
2006; pp. 160-165. cited by applicant .
Mortier; "An experimental study on the preparation of gold
nanoparticles and their properties"; May 2006, XP002576469,
Internet; 146 pages. cited by applicant .
Murphy et al.; "One-Dimensional Colloidal Gold and Silver
Nanostructures"; Inorganic Chemistry; vol. 45, No. 19; 2006; pp.
7544-7554. cited by applicant .
Nagai et al.; "Electric Conductivity-Filled Polymer Composites:
Orientation Control of Nanowires in a Magnetic Field"; ACS Appl.
Mater. Interfaces, vol. 3, No. 7; 2011; pp. 2341-2348--Abstract
only. cited by applicant .
Niidome et al; "Rapid synthesis of gold nanorods by the combination
of chemical reduction and photoirradiation processes; morphological
changes depending on the growing processes"; Chem. Commun.; 2003;
pp. 2376-2377. cited by applicant .
Pazos-Perez et al.; "Synthesis of Flexible, Ultrathin Gold
Nanowires in Organic Media"; Langmuir, vol. 24; 2008; pp.
9855-9860. cited by applicant .
Perez-Juste et al.; "Electric-Field-Directed Growth of Gold
Nanorods in Aqueous Surfactant Solutions"; Advanced Functional
Materials, vol. 14, No. 6; 2004; pp. 571-579. cited by applicant
.
Rathmell et al.; "The growth mechanism of copper nanowires and
their properties in flexible, transparent conducting films";
Advanced Materials, vol. 22; 2010; pp. 3558-3563. cited by
applicant .
Rathmell et al.; "The Synthesis and Coating of Long, Thin Copper
Nanowires to Make Flexible, Transparent Conducting Films on Plastic
Substrates"; Advanced Materials, vol. 23; 2011; pp. 4798-4803.
cited by applicant .
Reddy et al.; "Synthesis and cathodoluminescence properties of
CdSe/ZnO hierarchical nanostructures"; Journal of Materials
Chemistry; vol. 21; 2011, pp. 3858-3864. cited by applicant .
Stawinski et al., "Synthesis and Alignment of Silver Nanorods and
Nanowires and the Formation of Pt, Pd, and Core/Shell Structures by
Galvanic Exchange Directly on Surfaces"; Langmuir, vol. 23, No. 20;
2007; pp. 10357-10365; Abstract only. cited by applicant .
Taub et al.; "Growth of Gold Nanorods on Surfaces"; Journal Phys.
Chem. B, vol. 107, No. 45; pp. 11579-11582. cited by applicant
.
The Free Dictionary, definition of "several",
thefreedictionary.com, accessed Mar. 29, 2016, 4 pages. cited by
applicant .
Tvingstedt et al.; "Electrode Grids for ITO Free Organic
Photovoltaic Devices"; Advanced Materials, vol. 19; 2007; pp.
2893-2897. cited by applicant .
Wang et al., "Ultrathin Au Nanowires and Their Transport
Properties" J. Am. Chem. Soc., vol. 130, No. 28; 2008; pp.
8902-8903--Abstract only. cited by applicant .
Wang et al.; "Facile Synthesis of Ultrathin and Single-Crystalline
Au Nanowires"; Chemistry--An Asian Journal; Jul. 6, 2009; pp.
1028-1034--Abstract only. cited by applicant .
Wang, C.,"Ultrathin Au Nanowires and Their Transport Properties",
J. Am. Chem. Soc., 130, 8902-8903 (2008). cited by applicant .
Written Opinion for International Application No.
PCT/IL2013/050184; International Filing Date Feb. 28, 2013; dated
May 28, 2013; 8 pages. cited by applicant .
Zeng et al.; "A New Transparent Conductor: Silver Nanowire Film
Buried at the Surface of a Transparent Polymer"; Advanced
Materials, vol. 22; 2010; pp. 4484-4488. cited by applicant .
Zheng et al.; "Controlling synthesis of silver nanowires and
dendrires in mixed surfactant solutions"; Journal of Colloid
Interface Science, vol. 268; 2003; pp. 357-361. cited by applicant
.
Zhu et al.; "Reductive-Oxidation Electrogenerated Chemiluminescence
(ECL) Generation at a Transparent Silver Nanowire Electrode";
Langmuir, vol. 27, No. 6; 2011; pp. 3121-3127--Abstract only. cited
by applicant .
Zhu et al.; "Transparent Conductors from Carbon Nanotubes
LBL--Assembled with Polymer Dopant with .pi.-.pi. Electron
Transfer"; J. Am, Chem. Soc., vol. 133, No. 19; 2011; pp.
7450-7460--Abstract only. cited by applicant .
Zijlstra et al.; "High-Temperature Seedless Synthesis of Gold
Nanrods"; J. Phys. Chem. B; vol. 110; 2006; pp. 19315-19318. cited
by applicant .
Ziyang Huo, "Sub-two Nanometer Single Crystal au Nanowires" Nano
Letters (2008) vol. 8, No. 7 2041-2044. cited by applicant.
|
Primary Examiner: Dumbris; Seth
Attorney, Agent or Firm: Cantor Colburn LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of U.S. application
Ser. No. 13/061,745, filed on Mar. 2, 2011 which is a U.S. National
Stage of PCT International Application No. PCT/IL09/00842, filed on
Sep. 1, 2009, which claims priority to and benefit of domestic
filing of U.S. Provisional Application No. 61/190,712, filed on
Sep. 2, 2008, the disclosure of which is also incorporated herein
by reference in its entirety.
Claims
The invention claimed is:
1. A film comprising a plurality of nanowire bundles, each nanowire
bundle comprising a plurality of conductive metal nanowires, the
plurality of conductive metal nanowires comprising conductive metal
nanowires having cross-sectional diameters between 3 and 5 nm and
aspect-ratios of more than 500, wherein the conductive nanowires
being a composite of 85-90% gold and 10-15% silver.
2. The film according to claim 1, wherein said aspect-ratio is
greater than 1,000.
3. The film according to claim 1 comprising at least one
surfactant.
4. The film according to claim 1, formed on a substrate.
5. The film according to claim 1, wherein the nanowires are
distributed in a film of at least one surfactant.
6. A device comprising a film according to claim 1.
7. The device according to claim 6 being selected from an electrode
structure, being optionally configured as a photocathode, a
photocathode structure having an optically transparent substrate
carrying said film, an optically transparent electrode, an
electronic device having an electrodes' assembly, a transistor
device wherein at least one of source, drain and gate electrodes
comprises said film, a transistor device comprising a gate on an
insulator structure comprising an electrically insulating substrate
carrying said film, and an electroluminescent screen device
comprising a luminescent substrate structure carrying said film.
Description
FIELD OF THE INVENTION
This invention relates to the formation of metal nanowire
thin-films.
BACKGROUND OF THE INVENTION
Applications of thin, transparent and electrically conducting films
are numerous. The most attractive application is as a transparent
electrode for low-cost photovoltaics and other opto-electronic
applications. Light emitting devices often require such electrodes,
in particular, large area displays. Currently, the existing
technology uses conducting metal oxide films, primarily indium-tin
oxide (ITO) and doped zinc oxide for these applications. These
films have a limited transparency/conductivity trade-off and are
produced by expensive vacuum deposition techniques. Such films are
also hard and brittle and may therefore be unsuitable for flexible
coatings such as plastic electronics. A flexible alternative that
has been considered is a film of a conducting polymer, but such
films are much less conductive and more sensitive to radiation and
chemical attack and would thus not be good candidates for ITO
replacement.
In recent years, there is a growing interest in finding
alternatives for these transparent oxide electrodes. The primary
candidates have been carbon nanotube-based electrodes. However,
such films could not exceed the performance of ITO films in terms
of conductivity vs. visible light transmittance. There are several
problems in producing highly conductive, transparent carbon
nanotube mesh films. The limited solubility of the tubes makes it
difficult to disperse them in various solvents for efficient
coating applications. To produce such dispersions, high molecular
weight polymer surfactants are required, which produce an
insulating or semiconducting layer [1] around the nanotubes and
thus significantly increase the inter-tube contact resistance.
Another alternative is to use pure carbon nanotube meshes or
fabrics for this purpose, but here the nanotube density is too high
and optical transmission is degraded, and it is thus difficult to
integrate such meshes into thin-film devices. The same difficulties
hold for other types of prefabricated nanowires made of various
oxides and semiconductors.
Recently, thin conducting films consisting of high aspect ratio
nanostructures have been suggested as a substitute for metal oxide
based transparent electrodes, particularly in combination with
conducting polymer based devices [2, 3, 4]. Such films, made of
metallic nanowires have high conductivity while maintaining a metal
volume fraction as low as .about.1% and thus are highly
transparent.
Many schemes of synthesizing conducting and semi-conducting
nanowires were developed in the last 15 years. A very high control
level of the nanowires geometry and composition, including
modulation of compositions along or across the nanowires, has been
achieved. A control over the position and orientation of the
nanowire growth has been achieved in catalytic growth of carbon
nanotubes and semiconductor nanowires by positioning of the
catalyst particles at selected places. However, the task of forming
uniform thin films of such elongated nano-objects to obtain highly
conductive meshes over large areas has been more difficult to
achieve. High molecular weight polymeric surfactants are required
for dispersing the nanowires/nanotubes in various solvents. These
polymers usually form insulating barriers over the nanowires, which
would then require annealing to reduce inter-wire electrical
resistance [2], unless the polymer itself is (semi-)conducting [1,
4].
Peumans et al., have recently published a first calculation and
demonstration of a random silver nanorods mesh electrode as a
replacement for a metal oxide film in a polymer based solar cell
[2]. Peumans et al used prefabricated silver nanorods with an
average aspect ratio of .about.84, coated by a high molecular
weight polymer and dispersed in a solvent to prepare the thin
conductive film. The film required substantial annealing to reduce
the contact resistance between the nanorods, which probably was the
primary factor limiting the performance of this film. The film,
with comparable transmittance and conductivity to an ITO film,
exhibited 19% higher photocurrent when used for the polymer solar
cell compared to the ITO analog.
Gold nanowires have also been prepared in oleylamine, employing a
variety of methods.
REFERENCES
[1] US patent application No. 20080088219, Transparent carbon
nanotube electrode using conductive dispersant, Yoon, S. M. et al.,
13 Apr. 2007. [2] Lee, J-Y., Connor, S., T. Cui, Y., Peumans, P.,
Solution-Processed Metal Nanowire Mesh Transparent Electrodes, Nano
Lett. 8, 689-692 (2008). [3] Kang, M. G., Kim, M. S., Kim, J., Guo,
L. J., Organic Solar Cells Using Nanoimprinted Transparent Metal
Electrodes, Adv. Mater. 20, 4408-4413 (2008). [4] Hellstrom, S. L.,
Lee, H. W., Bao, Z., Polymer-Assisted Direct Deposition of Uniform
carbon nanotube bundle networks for high performance transparent
Electrodes, ACS Nano, 3, 1423-1430 (2009). [5] Lu, X., Yavuz, M.
S., Tuan, H-Y., Korgel, B. A., Xia, Y., Ultrathin Gold Nanowires
Can Be Obtained by Reducing Polymeric Strands of Oleylamin-AuCl
Complexes Formed via Aurophilic Interaction, J. Am. Chem. Soc. 130,
8900-8901 (2008). [6] Wang, C., Hu, Y., Lieber, C. M., Sun, S.,
Ultrathin Au Nanowires and Their Transport Properties, J. Am. Chem.
Soc., 130, 8902-8903 (2008). [7] Huo, Z., Tsung, C-K., Huang, W.,
Zhang, X., Sub-Two Nanometer Single Crystal Au Nanowires, Nano
Lett., 8, 2041-2044 (2008). [8] Pazos-Perez, N., Baranov, D.,
Irsen, S., Hilgendorff, M., Liz-Marazan, L. M., Giersing, M.,
Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media,
Langmuir, 24, 9855-9860 (2008). [9] Krichevski, O., Tirosh, E.,
Markovich, G., Formation of Gold-Silver Nanowires in Thin
Surfactant Solution Films, Langmuir 22, 867-870 (2006). [10]
Krichevski, O., Levi-Kalisman, Y., Szwarcman, D., Lereah, Y.,
Markovich, G., Growth of Au/Ag nanowires in thin surfactant
solution films: an electron microscopy study, J. Colloid Interface
Sci. 314, 304 (2007). [11] Krichevski, O., Markovich, G., Growth of
Colloidal Gold Nanostars and Nanowires Induced by Palladium Doping,
Langmuir 23, 1496-1499 (2007). [12] Jana, N. R., Gearheart, L.,
Murphy, C. J., Wet Chemical Synthesis of High Aspect Ratio
Cylindrical Gold Nanorods, J. Phys. Chem. B 105, 4065 (2001). [13]
Jana, N. R., Gearheart, L. A., Obare, S. O., Johnson, C. J., Edler,
K. J., Mann, S., Murphy, C. J., Liquid crystalline assemblies of
ordered gold nanorods, J. Mater. Chem. 12, 2909-2912 (2002).
SUMMARY OF THE INVENTION
The inventors of the present application, in their pursuit to
improve on the deficiencies of the art have developed a finer,
higher aspect-ratio (above 1000) homogeneous highly conductive mesh
of metal nanowires. The manufacture of these nanowires employed a
solution-process whereby the metal nanowires are formed from a
solution of metal precursors, which slowly dries into a mesh of
nanowires with controllable surface coverage. These metal nanowires
made of a metal such as gold, silver, copper, nickel, palladium,
and combinations thereof, significantly out-perform many of the
nanowires known in the literature, including the silver nanorod and
ITO films of the art, by having at least one order of magnitude
better conductivity, for visible light transmission levels of
80-90% regularly achieved in such films, e.g., ITO films.
This invention is, thus, generally concerned with a process for the
preparation of a conductive nanowire film (herein referred to as a
nanowire film) based on a high aspect-ratio metal, e.g., gold
nanowires. The nanowire film is produced by inducing metal
reduction in a concentrated surfactant solution containing metal
precursor ions, at least one surfactant and at least one reducing
agent, forming a thin-film thereof on a surface of a substrate and
allowing the film to dry. The metal nanostructures begin forming in
the concentrated surfactant solution that progressively becomes
more concentrated as the film dries.
The nanowire film, thus obtained, has metallic conductivity and
high transparency to light due to the low volume filling of the
metal in the film. These nanowire films find use as transparent
electrodes for photovoltaic and other opto-electronic devices
(e.g., photovoltaic and light emitting diode devices), as will be
further disclosed hereinbelow. The processes of the invention for
manufacturing nanowire films are suitable for printing conducting
patterns on various surfaces using a great variety of techniques
such as ink jet printing.
Thus, in one aspect of the present invention there is provided a
process for the preparation of a nanowire film on a surface of a
substrate, said process comprising:
(a) obtaining an aqueous precursor solution comprising at least one
metal precursor, at least one surfactant and at least one metal
reducing agent wherein the concentration of the at least one
surfactant in said solution is at least 5% (w/w);
(b) forming a thin-film of the precursor solution, i.e., of step
(a), on at least a portion of a surface of a substrate; and
(c) allowing nanowires to form in said thin-film, on a portion of
said surface, e.g., by allowing the thin film to dry;
thereby obtaining a nanowire film on at least a portion of said
surface.
In some embodiments, the nanowire is conductive.
In certain embodiments, the process of the invention comprises a
step of pre-treating the surface of the substrate to prepare it to
better receive the deposition of the solution.
The pre-treatment may include, in a non-limiting fashion, solvent
or chemical washing (e.g., by a non-liquid medium such as a gas),
etching, heating, deposition of an optionally patterned
intermediate layer to present an appropriate chemical or ionic
state to the nanowire formation, as well as further surface
treatments such as plasma treatment, UV-ozone treatment, or corona
discharge.
In certain embodiments, the process further comprises the step of
post-treating the resulting conductive nanowire film. In some
embodiments, the post-treatment involves at least one of washing
the conductive nanowire film with an aqueous or organic liquid or
solution to e.g., remove excess surfactant, and thermally treating
the film, e.g., at a temperature not exceeding 100.degree. C.
The aqueous solution comprising the at least one metal precursor,
at least one surfactant and at least one metal reducing agent,
herein referred to as the precursor solution, may be prepared by
forming a solution or a mixture (by mixing, admixing) of the
components together at a temperature which permits complete
dissolution of the components in each other or in an inert medium
(such as water), permitting formation of a substantially homogenous
solution. It should be noted, that the term "solution" should be
given its broadest definition to encompass a liquid state in which
one component is fully dissolved in another or in a liquid medium,
a liquid state of emulsion (nano- or microemulsion) of one or more
components of the precursor solution in another or in a medium, and
a state of dispersion (nano- or microdispersion) of one or more
components of the precursor solution in another or in a medium. In
some embodiments, the precursor solution is a homogenous nano- or
micro-emulsion.
The precursor solution is prepared, in some embodiments, by
combining (mixing, admixing) the components at room temperature. In
some other embodiments the mixing is carried out at a temperature
above room temperature, e.g., in different embodiments the
temperature is between 25-100.degree. C., between 25-75.degree. C.,
between 30-50.degree. C., between 30-40.degree. C., between
40-75.degree. C., or between 50-75.degree. C.
In some embodiments, the precursor solution is prepared by first
forming a solution of at least two of the components, e.g., the at
least one first metal precursor and at least one surfactant at a
temperature allowing dissolution of one component in the other, or
both components in an inert medium such as water (or another medium
which permits their dissolution or emulsification), followed by the
addition (e.g., by way of admixing) of the at least one other
component, e.g., reducing agent and/or at least one second metal
precursor, while maintaining the temperature so as to sustain a
substantially homogenous solution.
In some embodiments, the aqueous precursor solution is prepared by
first forming a solution of at least one first metal precursor, at
least one surfactant and at least one second metal precursor at a
temperature allowing dissolution, followed by the addition of at
least one reducing agent.
In some embodiments, the aqueous precursor solution is prepared by
first forming a solution of at least one first metal precursor, at
least one surfactant, at least one reducing agent and at least one
second metal precursor at a temperature allowing dissolution,
followed by the addition of at least one second reducing agent.
The process of the invention is suitable for the preparation of a
great variety of conductive metal nanowires. The metal nanowires
may be of gold, silver, copper, nickel, palladium, platinum or
combinations thereof. The at least one metal precursor is thus a
metal precursor containing the element (in any form, e.g., ionic or
non-ionic) making up the nanowire. Typically, the metal precursor
is in the form of metal ions or in a form which under the reaction
conditions dissociates into metal ions. Non-limiting examples of
metal precursors are chloroauric acid, HAuCl.sub.4, as a source of
gold; AgNO.sub.3 as a source of silver; (NH.sub.4).sub.2PdCl.sub.6
as a source of palladium; Cu(NO.sub.3).sub.2 as a source of copper;
NiCl.sub.2 as a source of nickel; and H.sub.2PtCl.sub.6 as a source
of platinum.
In some embodiments, the at least one metal precursor is a single
metal precursor. In other embodiments, the at least one metal
precursor is a combination of two or more metal precursors or the
same metal or of different metals.
In some embodiments, the metal precursor is a gold precursor, such
as chloroauric acid. In other embodiments, the metal precursor is a
combination of gold and silver metal precursors. In still further
embodiments, the metal precursor is a combination of palladium,
silver and/or gold metal precursors.
The metal precursor concentration is about at least 1 mM. In some
embodiments, the concentration is between 1 and 15 mM. In other
embodiments, the concentration is between 1 and 10 mM.
The at least one surfactant may be a single surfactant or a mixture
of two or more surfactants. The at least one surfactant is
typically selected amongst cationic-type surfactants, typically
quaternary ammonium based molecules, such as those having at least
one alkyl chain of 10 or more carbon atoms; in some embodiments of
at least 14 carbon atoms, e.g., 14, 16 or 18 carbon atoms. In some
embodiments, the at least one surfactant has one alkyl chain of
between 14 and 16 carbon atoms. In other embodiments, the at least
one surfactant is a multi-chain surfactant having two or more alkyl
chains, each of between 10 and 16 carbon atoms.
Non-limiting examples of said surfactant are cetyltrimethylammonium
bromide (CTAB), didodecyldimethylammonium bromide,
tetradecyltrimethylammonium bromide, didecyldimethylammonium
bromide, wherein the bromide counterion, alternatively, may be a
chloride or an iodide.
In some embodiments, the concentration of the at least one
surfactant is above 5%, in further embodiments above 10%, in still
other embodiments above 15%, and in yet other embodiments, the
concentration is above 20%. In some additional embodiments, the
surfactant concentration is at most 30%. In additional embodiments,
the surfactant concentration is between 7.5 and 21%.
It should be noted that where various embodiments are described by
using a given range, the range is given as such merely for
convenience and brevity and should not be construed as an
inflexible limitation on the scope of the invention. Accordingly,
the description of a range should be considered to have
specifically disclosed all the possible sub-ranges as well as
individual numerical values within that range. For example, an
alkyl chain having between 10 and 16 carbon atoms should be
considered to have specifically disclosed sub-ranges such as from
10 to 13, from 10 to 14, from 10 to 15, from 11 to 13, from 11 to
14, from 11 to 15, from 11 to 16, from 12 to 14, from 12 to 15,
etc., as well as individual numbers within that range, for example,
10, 11, 12, 13, 14, 15, and 16.
The at least one reducing agent employed in the precursor solution
is an agent capable of reducing the at least one first and/or
second metal precursors. In some embodiments, the metal reducing
agent is inorganic and in other embodiments, the metal reducing
agent is an organic agent. Non-limiting examples of such reducing
agents are metal borohydride, e.g., such as sodium borohydride and
other hydride derivatives, such as cyanoborohydride, sodium
ascorbate, hydroquinone and hydroquinone derivatives, hydrazine and
hydrazine derivatives, such as methylhydrazine and any combinations
thereof.
In some embodiments, the at least one reducing agent is two or more
agents introduced into the reaction mixture at the same time or at
different times. In some embodiments, the two or more reducing
agents are different in their reducing ability; the first may be a
weak reducing agent such as sodium ascorbate and the second may be
a strong reducing agent such as a metal borohydride.
As the process of the method recites, upon formation of the
precursor solution, the solution or an aliquot thereof is placed on
at least a portion of the surface to be coated (which had
optionally been pre-treated) and a thin-film is allowed to form
thereon. To enable the deposition of the precursor solution with a
controlled thickness and uniformity over the surface, different
techniques may be employed depending on the size of the surface,
its structure, viscosity of the solution (as derived for example by
the specific surfactant concentration), the temperature of the
precursor solution, and other parameters as may be known to a
person skilled in the art. Generally, for lower surfactant (low
viscosity) solutions containing 1-15% surfactant (w/w) spray
coating may be used, by employing, for example, a spray coating
system having a high pressure nebulizer and a temperature
controlled substrate holder. Such precursor solutions may also be
applied onto the surface by employing the ink-jet printing
technology and the roller printing technique known in the art. For
higher surfactant concentrations of above 15% (w/w), drop casting,
dip- and spin-coating techniques and roller printing techniques are
also suitable for covering large surfaces.
The thickness of the thin-film depends on the viscosity, as
determined by the surfactant concentration and temperature, of the
precursor solution. Notwithstanding, the spread thickness typically
employed is between 10 and 100 .mu.m.
The surface of a substrate or an object on which the thin-film is
formed according to the process of the present invention may be of
any rigid or flexible substrate or object. The substrate can be
clear (transparent; any degree of transparency) or opaque. The
surface may be hydrophobic or hydrophilic in nature (or at any
degree of hydrophobicity/hydrophilicity or a surface which may be
switched between the two states). The surface may be an organic or
inorganic surface such as a silicon surface (such as a standard,
polished silicon wafer), a fused silica surface (such as a standard
fused silica window polished to optical quality), a carbon surface
(such as a highly oriented pyrolitic graphite), a surface of a
relatively smooth polymer sheet (such as polycarbonate copying
machine transparency film and a semiconducting polymer layer
comprising the active layer of an organic light emitting diode
made, for example from MEH-PPV or doped polyacetylene), and any
other surface.
The surface may be whole surface or a portion thereof. The portion
(region) of the substrate's surface to be coated may be of any size
and structure, the portion may be continuous or comprise of several
non-continuous sub-regions on the surface. In some embodiments, the
surface of the substrate is substantially two-dimensional. In other
embodiments, the surface is that of a three-dimensional object. In
other embodiments, the at least one portion of the substrate's (or
object's) surface is its whole surface.
Once the surface is covered, partially or wholly, with a thin-film
of the precursor solution, it is allowed to dry. Unlike processes
of the art, the drying of thin-film produced by the process of the
present invention does not require high temperature. In some
embodiments, the thin-film of the invention is allowed to dry at
ambient temperature, i.e., between 25-27.degree. C. In other
embodiments, drying is achieved at a temperature not exceeding
40.degree. C. In further embodiments, drying is achieved at a
temperature between 27-40.degree. C. In still other embodiments,
drying is achieved at a temperature between 35-40.degree. C.
The drying period does not typically exceed 60 minutes. In some
embodiments, the thin-film is dried over a period of between 30-60
minutes, in other embodiments, between 30-45 minutes and in further
embodiments over a period of up to 30 minutes (e.g., 1, 5, 7, 10,
15, 17, 20, 22, 25, 27 minutes or any period therebetween).
In some embodiments, nanowire formation may be induced (initiated),
accelerated or generally controlled (controlling the morphology of
the nanowires, their formation, their length, aspect ratio, bundle
formation, accelerating their formation, arresting their formation,
etc) by irradiating the film of the precursor solution (on at least
a portion of the surface of a substrate) with ultraviolet light
(UV). In some embodiments, the film is irradiated with a UV light
at 254 nm (e.g., mercury lamp). The exposure duration may be from a
few seconds to a few hours depending on the thickness of the film,
the concentration of the surfactant, the temperature of the film,
the size of the substrate and other factors.
In some embodiments, the film is exposed to a 100 W mercury lamp,
in some embodiments, for 1-30 minutes.
The processes of the invention may be compatible with large scale
deposition techniques, such as roll-to-roll printing. This process
may allow better control of the nanowire dimensions and densities
as well as reduction of the residual spherical nanoparticle
population which is detrimental for the optical transmission
properties of the films. Due to the directionality of the nanowire
bundles of the invention, as will be further disclosed hereinbelow,
they may be aligned using various alignment techniques used for
liquid crystals, such as the use of external fields or shear
forces. Such aligned nanowire arrays may be useful for future
nanoelectronic circuits.
The present invention, in another of its aspects provides an
aqueous solution (e.g., the homogeneous precursor aqueous solution,
emulsion or dispersion) comprising at least one metal precursor
(e.g., at least one salt of a metal selected from gold, silver,
copper, palladium, platinum or a mixture thereof), at least one
surfactant and at least one reducing agent, wherein the
concentration of the at least one surfactant in said solution is at
least 5% (w/w), and wherein each of the components is as defined
hereinabove.
In some embodiments, the concentration of the at least one
surfactant is above 5%, in further embodiments above 10%, in still
other embodiments above 15%, and in yet other embodiments, the
concentration is above 20%. In some additional embodiments, the
surfactant concentration is at most 30%. In additional embodiments,
the surfactant concentration is between 7.5 and 21%.
In some embodiments, the medium is water, preferably pure water,
e.g., double distilled, triply distilled, or ultra-pure. In other
embodiments, the at least one metal precursor is gold and/or
silver.
In still other embodiments, the solution (e.g., precursor solution)
of the invention is at a temperature at which the solution is
substantially homogeneous. Such a temperature, as disclosed above,
may be ambient or a higher temperature.
In some embodiments, where the at least one metal reducing agent is
sodium ascorbate, the surfactant concentration is at least 1%.
In further embodiments, the solution of the invention is for use in
a process for the preparation of a conductive thin-film as
disclosed herein.
In another aspect of the present invention, there is provided a
process for the preparation of a nanowire film on a surface of a
substrate, said process being independent of a surfactant
concentration, said method comprising:
(a) obtaining an aqueous precursor solution, said solution being
prepared by: (i) combining (forming a solution of) at least one
surfactant, at least one (first) metal precursor and at least one
metal reducing agent in an aqueous medium; (ii) inducing metal
reduction of said at least one metal precursor;
(b) forming a thin-film of the solution of step (a) on at least a
portion of a surface of a substrate; and
(c) allowing nanowires in said thin-film to form, e.g., by drying
the thin film;
thereby obtaining a nanowire film (e.g., a conductive film) on at
least a portion of said surface.
In some embodiments, the reduction of the at least one first metal
precursor (a first metal precursor) is induced by the addition of
at least one second metal precursor. In some embodiments, said at
least one second metal precursor is a silver metal precursor.
In further embodiments, the at least one first metal precursor is a
gold metal precursor and the aqueous precursor solution is obtained
by: (i) forming a solution of at least one surfactant, at least one
gold metal precursor and at least one metal reducing agent in an
aqueous medium; (ii) adding into the aqueous solution at least one
silver metal precursor to thereby induce reduction of said at least
one gold metal precursor.
In other embodiments, the metal reducing agent is sodium
ascorbate.
In yet additional embodiments of this process of the invention, the
concentration of said at least one surfactant is between 1% and 10%
(w/w) of the total weight of the precursor solution. In some
embodiments, the concentration is between 1% and 5%. In other
embodiments, the concentration is between 1% and 3%. In other
embodiments the concentration is between 1% and 2%. In still
further embodiments, the surfactant concentration is 1.6%
(w/w).
There is thus provided a process for the preparation of a
conductive nanowire film on a surface of a substrate, said process
comprising:
(a) obtaining an aqueous precursor solution, said solution being
prepared by: (i) forming a solution of at least one surfactant at a
concentration of between 1-10% (w/w), at least one gold metal
precursor and sodium ascorbate in an aqueous medium; (ii) adding at
least one silver metal precursor
(b) forming a thin-film of the solution of step (a) on at least a
portion of a surface of a substrate; and
(c) allowing said thin-film to dry;
thereby obtaining a gold/silver nanowire film on at least a portion
of said surface.
As stated hereinabove, in some embodiments, the precursor solution
is prepared by first forming a solution of at least one first metal
precursor, at least one surfactant and at least one second metal
precursor at a temperature allowing dissolution, followed by the
addition of at least one reducing agent. In such embodiments, the
process of the invention, being independent of a surfactant
concentration, comprises:
(a) obtaining an aqueous precursor solution, said solution being
prepared by: (i) combining (forming a solution of) at least one
surfactant, at least one first metal precursor, at least one
reducing agent, and at least one second metal precursor in an
aqueous medium; (ii) introducing at least one second reducing
agent, to induce reduction of said at least one first metal
precursor;
(b) forming a thin-film of the solution of step (a) on at least a
portion of a surface of a substrate; and
(c) allowing said thin-film to dry;
thereby obtaining a nanowire film on at least a portion of said
surface.
In these embodiments, the reduction of the at least one first metal
precursor is induced by the addition of the at least one second
reducing agent after a solution has been formed of the first and
second metal precursors and first reducing agent and the at least
one surfactant.
The reducing agents employed are typically a hydride or metal
borohydride and sodium ascorbate.
In further embodiments, the at least one first metal precursor is a
gold metal precursor and the at least one second metal precursor is
silver.
As above, the concentration of said at least one surfactant is
between 1% and 10% (w/w) of the total weight of the precursor
solution. In some embodiments, the concentration is between 1% and
5%. In other embodiments, the concentration is between 1% and 3%.
In other embodiments the concentration is between 1% and 2%. In
still further embodiments, the surfactant concentration is 1.6%
(w/w).
There is thus provided a process for the preparation of a
conductive nanowire film on a surface of a substrate, said process
comprising:
(a) obtaining an aqueous precursor solution, said solution being
prepared by: (i) forming a solution of at least one surfactant at a
concentration of between 1-10% (w/w), at least one gold metal
precursor and at least one silver metal precursor and at least one
ascorbate reducing agent in an aqueous medium; (ii) adding a metal
borohydride;
(b) forming a thin-film of the solution of step (a) on at least a
portion of a surface of a substrate; and
(c) allowing said thin-film to dry;
thereby obtaining a gold/silver nanowire film on at least a portion
of said surface.
The at least one surfactant employed with this process of the
invention, is as defined hereinabove.
In some embodiments of all processes of the invention, the at least
one surfactant is one comprising at least one quaternary ammonium
group.
The invention also provides a kit comprising, in the same container
or in different containers, at least one metal precursor, at least
one surfactant and at least one reducing agent, optionally a liquid
medium (such as water), means to permit dissolution of each of the
components of the kit in each other or in the medium, and
instructions to prepare a precursor solution. Where two or more
metal precursors are to be used, the kit may comprise each in a
separate container.
As used herein, the process, the precursor solution or the kit of
the invention may include additional steps or ingredients or parts,
only if the additional steps, ingredients, or parts do not alter
the basic and novel characteristics of the claimed process,
solution and kit.
As used herein, the singular form "a", "an" and "the" include
plural references unless the context clearly dictates otherwise.
For example, the term "a metal precursor" or "at least one metal
precursor" may independently include a plurality of metal
precursors, including mixtures thereof.
As used herein, the term "metal nanowire" refers to a continuous
metallic wire comprising one or more elemental metal, a metal alloy
thereof and, in some embodiments, a metallic compound, e.g., a
metal oxide thereof. The cross sectional diameter of the metal
nanowire is less than 100 nm. In some embodiments, the
cross-sectional diameter is less than 50 nm, in other embodiments
less than 10 nm and in further embodiments the diameter is between
2-10 nm, or 2-9 nm, or 2-8 nm, or 2-7 nm, or 2-6 nm, or 2-5 nm, or
2-4 nm, or 3-5 nm.
The metal nanowire has an aspect-ratio (the ratio of length of the
nanowire to its width) greater than 100, in some embodiments
greater than 500, and further embodiments greater than 1000. As
used herein, "high aspect ratio" refers to an aspect ratio above
100.
In some embodiments, the metal nanowires are of a metal selected
from gold, silver, copper, nickel, palladium or combinations
thereof. In some other embodiments, the metal nanowires are of gold
and silver.
As stated above, the process of the invention provides for the
preparation of a nanowire film which comprises a plurality of such
metal nanowires. In some embodiments, the nanowire film comprises a
plurality, i.e., at least two, metal nanowires arranged in the film
as separate nanowires randomly distributed and having a random
spatial arrangement, and/or in groups or bundles of two or more
nanowires, with each nanowire in the bundle substantially having
the same spatial arrangement and direction. The independent
nanowires or bundles thereof may also take on the form of a mesh,
namely a formation of closely spaced and randomly crossing metal
nanowires, said mesh being conductive throughout, namely at any two
arbitrary points thereof.
Since the nanowire film comprises of a sufficient number of metal
nanowires, electrical conductivity is observed with electrical
charge percolation from one metal nanowire to another. The nanowire
film has thus electrical conductivity.
As one versed in the art would recognize, the conductivity referred
to is a metallic electrical conductivity or ohmic electrical
conductivity, i.e., exhibiting linear current/voltage curves. In
some embodiments, the film conductivity does not exceed 1000 Ohm
square.
The nanowire film additionally has high transparency to light due
to the low volume filling of the metal in the film, namely the
occupation of overall only a small surface area by the nanowires.
The nanowire film is transparent between 400-800 nm. In some
embodiments, the light transmission of the nanowire film is at
least 75% and can be as high as 98%. In some further embodiments,
the transmission is between 75-85%.
The metal nanowires and the conductive nanowire films of the
invention may be fabricated into substantially any device that can
utilize such nanostructures or articles associated therewith. Such
nanostructures and articles of the invention can be used in a
variety of applications, such as sensors (such as electrochemical
sensors, mechanical sensors, electromechanical sensors), tags or
probes, electrodes (such as transparent electrodes), switches,
transistors, displays, photovoltaic cells and other opto-electronic
devices.
The structural, chemical and electronic properties of the specific
metal nanowire or film may be used in the design and manufacture of
a variety of such devices. For some applications, the metal
nanowires or films are integrated into a functional component of a
device for use, in some non-limiting examples, in surface-enhanced
Raman scattering, subwavelength optical waveguiding, biolabeling,
and biosensing, particularly where the nanowires of composed or
gold and/or silver metals.
For other applications, the metal nanowires of the invention and
the film comprising same, may be further functionalized to impart
to the film certain surface properties. Functionalization of the
conductive nanowire film of the invention may be through
functionalization of the metal nanowires or through
functionalization of the exterior surfaces of the film.
The invention thus provides an electrode structure comprising an
electrically conductive film comprising a plurality of electrically
conductive nanowires on a substrate, which may or may not be
optically transparent. In some embodiments, the electrode structure
is configured as a photocathode. In other embodiments, the
substrate is optically transparent. The film comprising said
plurality of electrically conductive nanowires according to the
invention may be a portion of a substrate.
The invention further provides a photocathode structure comprising
an optically transparent substrate carrying a layer formed by an
arrangement (e.g., a mesh) of the conductive nanowires.
An optically transparent electrode is also provided, said electrode
comprising a conductive layer, according to the invention, formed
by an arrangement of the conductive nanowires on an optically
transparent substrate.
The invention also provides an electronic device comprising an
electrodes' assembly wherein at least one of the electrodes
comprises a conductive layer comprised of an arrangement of
conductive nanowires according to the invention on a substrate. In
some embodiments, the electronic device is configured and operable
as a marker (e.g., unique random pattern of wires having unique
distribution/profile of electric and/or magnetic field along the
substrate); a sensor (photodetector); a switch (transistor) and
other related devices. The electrodes' assembly may be selected
from a diode, triode, transistor, etc.
There is thus provided a transistor device wherein at least one of
source, drain and gate electrodes comprises the electrically
conductive layer of the conductive nanowires of the invention, on a
substrate.
A transistor device is also provided, wherein the device comprises
a gate on insulator structure having an electrically insulating
substrate carrying a conductive layer of electrically conductive
nanowires according to the invention.
The present invention also provides an electroluminescent screen
device comprising a luminescent substrate structure carrying a
layer of conductive nanowires according to the invention.
For some applications it may be necessary to embed the nanowire
film in a solid matrix, with portions of the nanowires extending
from the matrix to enable access to a conductive network. Such a
matrix may provide protection to the nanowires from adverse factors
such as corrosion and abrasion. The matrix may also offer
mechanical properties to the conductive nanowire layer.
Additionally, performance-enhancing layers may be used to further
enhance the characteristics of the nanowire film. This, for
example, may be achieved by introducing additional layers in the
transparent conductor structure of the invention. Thus, in other
embodiments, the invention also provides a multi-layer transparent
conductor which comprises the conductive nanowire film of the
invention and at least one additional layer selected from an
anti-reflective layer, an anti-glare layer, an adhesive layer, a
barrier layer, and a protective coat.
The invention thus provides a transparent conductor comprising a
substrate and a conductive film on at least a portion of a surface
of said substrate, the conductive film comprising a plurality of
metal nanowires as disclosed herein, and optionally at least one
performance enhancing layer, as disclosed.
In some embodiments, the nanowire conductive film is used for
multiple conductors in an integrated circuit chip.
For certain applications, the nanowire film may be treated, during
manufacture or after it has been formed with a polymeric surfactant
such as a cationic polymeric surfactant, so as to endow the
nanowires or the film as a whole with increased physical stability.
In some embodiments, the polymeric surfactant is
poly-diallyldimethylammonium chloride. Alternatively, polymerizable
monomers, such as styrene, that can be polymerized after film
drying and nanowire formation using a polymerization initiator
solution may be employed.
It should be appreciated that certain embodiments of the invention,
which are, for clarity, described as distinct embodiments, may also
be provided in combination in a single embodiment. Conversely,
various features of the invention, which are, for brevity,
described in the context of a single embodiment, may also be
provided separately or in any suitable combination or as suitable
in any other described embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to understand the invention and to see how it may be
carried out in practice, embodiments will now be described, by way
of non-limiting example only, with reference to the accompanying
drawings, in which:
FIGS. 1A-1B are transmission electron microscope (TEM) images of a
dried growth solution containing 7.5% CTAB deposited on a carbon
coated copper grid after washing most of the CTAB with water and
ethanol.
FIGS. 2A-2B are scanning electron microscope (SEM) images of
thin-films prepared from the 7.5% growth solution deposited on
(FIG. 2A) fused silica and (FIG. 2B) Si with native oxide
10.times.10 mm substrates.
FIG. 3 presents a visible light transmission curve of a film
deposited from a 7.5% CTAB solution on a fused silica substrate.
This film had a .about.500 .OMEGA.sq resistance.
FIG. 4 depicts a film prepared according to a process of the
art.
FIG. 5A-5D are TEM images of nanowire films deposited on carbon
coated grids: FIGS. 5A-5B show TEM obtained from 0.25 M CTAB
solution. The inset in FIG. 5A shows the uniformity of the nanowire
film over a macroscopic area (>100 .mu.m.sup.2 area). FIGS.
5C-5D show TEM obtained from 0.6 M CTAB solution. The inset in FIG.
5C shows a case where silver ion concentration was too low relative
to the CTAB 0.6 M concentration and the small nucleated segments
could not connect.
FIG. 6 shows a SEM image of a part of a nanowire film prepared on a
silicon substrate and washed with 70% ethanol.
FIGS. 7A-7D show SEM image of nanowire bundle, a typical
measurement configuration and current-voltage curves: FIG. 7A shows
the current-voltage measurement for the SEM image shown in FIG. 7B
of nanowire bundle conductance measurement using clean tungsten
nanoprobes in the Zyvex S100 system. FIG. 7C shows a typical
measurement configuration with the nanowire film deposited on a
pre-patterned silicon substrate with gold electrodes. FIG. 7D shows
current-voltage curves measured with various inter-electrode
spacings as indicated in the legend.
FIGS. 8A-8C show visible light transmission curve of a nanowire
film, bending of PET substrate coated with a nanowire film and the
periodic table as observed through the PET film. FIG. 8A shows a
visible light transmission curve of a nanowire film deposited on a
fused silica substrate with a sheet resistance of 200 .OMEGA./sq.
FIG. 8B demonstrates bending of a PET substrate coated with a
nanowire film to a curvature radius of .about.1.5 cm maintains a
.about.100 .OMEGA./sq sheet resistance. FIG. 8C displays the
periodic table as observed through the same PET film which had
.about.80-85% optical transmission in the visible range. The bright
stripes are silver paint lines used to estimate the sheet
resistance. The upper right corner is film-free.
DETAILED DESCRIPTION OF EMBODIMENTS
General Experimental Procedures
The preparation of a high aspect-ratio metal nanowire mesh films
with high conductivity, flexibility and transparency was based on
an in-situ formation of the nanowires which occurred after the
deposition of a thin film of precursor solution on top of a
substrate of choice.
Gold-silver nanowires were grown in a drying thin film containing a
high cationic surfactant concentration which formed a
liquid-crystalline template phase for the formation of a nanowire
network. The nanowire network films were uniform over macroscopic
(cm.sup.2 scale) areas and on a variety of substrates. These films,
measuring only few nanometers in thickness were characterized by
low sheet resistivities, in the range of 60-300 .OMEGA./sq, as
formed, and a high transparency, comparable to indium tin oxide
(ITO) films.
One process for the preparation of the metal nanowire mesh films
begins with the preparation of a relatively concentrated surfactant
solution having at least 5%, or at least 7.5%, or from 5% to 30%,
or from 5% to 21%, or from 7.5% to 21% (w/w) of a surfactant such
as cetyltrimethylammonium bromide (CTAB) in ultrapure water. The
formerly published process [9, 10] had only 1.6% CTAB. Such high
concentrations require heating of the solution so as to produce a
uniform micro-emulsion phase of the surfactant/water mixture.
A solution of chloroauric acid was added to this emulsion to yield
a final Au precursor concentration of between 1 and 4 mM and a
higher concentration of sodium ascorbate was added at a
concentration of 40 to 60 times higher than the gold concentration.
The initiation of the metal deposition process occurred by adding a
concentrated AgNO.sub.3 solution to the prepared solution at
30-40.degree. C., while stirring, to a final silver concentration 2
times higher than that of the gold. The silver ions added were
being reduced by the ascorbate ions and when small silver metal
seeds formed, the reduction of gold ions by the ascorbate was
catalyzed and the metal nanostructures began growing. Immediately
after silver addition a thin-film of the solution was spread on the
substrate of choice either by drop casting, dip-coating or
spin-coating. The thickness of such a film depended on the
viscosity (determined by surfactant concentration and temperature)
and the spread conditions and was measured to be between 10 and 100
.mu.m. Next, the film was dried, in some cases by placing the
substrate under mild heating by a lamp at 35-40.degree. C. until
the film fully dried, after about 10 minutes.
For microscopy studies of the dried films, most of the surfactant
was washed out with various solvents. For conductance measurements,
a quick ethanol wash was sufficient to allow for good electrical
contact, either to pre-fabricated electrodes patterned on the
substrate or to electrodes patterned post-film-deposition, either
by metal evaporation or by spreading silver paint on the film.
In another process according to the invention, the aqueous solution
was first formed by preparing a relatively concentrated surfactant
solution having at least 5%, or at least 7.5%, or from 5% to 30%,
or from 5% to 21%, or from 7.5% to 21% (w/w) of a surfactant such
as cetyltrimethylammonium bromide (CTAB) in ultrapure water. A
solution of chloroauric acid was added to this emulsion together
with a concentrated AgNO.sub.3 solution, while stirring, to a final
silver concentration 2 times higher than that of the gold. After a
few minutes, a solution of sodium borohydride was added followed by
a solution of sodium ascorbate. The silver and gold ions in the
presence of the strongly reducing agent began undergoing reduction,
forming silver/gold metal seeds, the reduction of gold and silver
ions by the ascorbate was catalyzed and the metal nanostructures
began growing.
Example 1
A 8.3% (w/w) cetyltrimethylammonium bromide (CTAB) solution was
prepared by heating and stirring the CTAB/water mixture at
50.degree. C. for 5 minutes. To this solution, at 40.degree. C.,
500 .mu.L of 25 mM HAuCl.sub.4 solution and 425 .mu.L of 1.8 M
freshly prepared sodium ascorbate solution were added and stirred
together. Then, 250 .mu.L of 100 mM AgNO.sub.3 solution were added
while stirring. The final CTAB concentration in the nanowire growth
solution was .about.7.5%. 30 seconds after the addition of the Ag
solution, the stirring was discontinued and the solution was
deposited on a substrate and let dry for 15-45 minutes at
35-40.degree. C.
Transmission electron microscopy (TEM) and scanning electron
microscopy (SEM) imaging revealed metal nanowire networks of
varying wire densities and entanglement, depending on the exact
solution and deposition conditions, uniformly spread over the
substrates (See, FIGS. 1A to 1B showing transmission electron
microscope images, and FIGS. 2A to 2B showing scanning electron
microscope images). The nanowires were typically 3-5 nm wide and
many micrometers long and in this case composed of 85-90% gold and
15-10% silver. A varying amount of non-elongated metal
nanostructures was also observed. The minimization of the
concentration of such structures in the films was a key to
improving their optical transmission.
Nanowire films were obtained from solutions that had up to 21% CTAB
concentrations, 5 mM HAuCl.sub.4, 0.2M sodium ascorbate and 10 mM
AgNO.sub.3. These concentrated CTAB solutions were highly viscous
and required longer mixing and heating times to prepare homogeneous
solution thereof. With such growth solutions it was easy to coat
the substrates by simple dip-coating.
The composition of the substrate did not influence the final
results since the high surfactant concentration ensured proper
wetting of either hydrophobic or hydrophilic surfaces. So far, the
process produced similar results on silicon, fused silica,
polycarbonate and carbon substrates. Differences between various
substrates were mostly due to edge effects of the drying film which
were more substantial in cases of small substrates such as TEM
grids. The high level of uniformity and thus nanowire percolation,
as seen in FIGS. 1A to 1B and 2A to 2B could not be obtained using
the procedure described in the art [e.g., in refs. 9 and 10].
Electrical measurements, done on several length scales at various
arbitrary positions on the substrates using various types of
contacts have shown ohmic conductance of the order of 100-500
.OMEGA./sq and 75-85% transmittance in the visible range (FIG. 3),
which is comparable to indium tin oxide (ITO) films. The estimated
conductivity per Au/Ag wire was of the order of bulk gold
conductivity. It should be noted that a significant part of the
.about.20% extinction observed in these experiments came from light
scattering that the simple spectrophotometer used for these
measurements could not collect, while in a thin-film photovoltaic
device most of the scattered light would be collected. Thus, the
total transmitted light was probably significantly higher than the
observed average .about.80%.
Contrary to the metal nanowire films prepared by the processes of
the invention, films prepared by the methods of the art,
particularly those described in references [9 and 10] do not result
in the formation of mesh film arrangements of the type observed in
FIGS. 1A to 1B and disclosed herein. In fact, and as FIG. 4
demonstrates, the previously published procedure typically yield a
film of spherical nanoparticles rather than a film of nanowires on
scaling up surfactant and reagent concentrations. The process of
the invention reproducibly yields metal nanowire films.
Example 2
A solution comprising surfactant cetyltrimethylammonium bromide
(CTAB), chloroauric acid, as a gold precursor at a molar ratio of
1:200 relative to the CTAB concentration, and sodium ascorbate, at
a molar ratio of 60:1 relative to the gold concentration, was
prepared. The nanowire growth solutions had CTAB concentrations of
0.25 M and 0.6 M, significantly higher than the 0.1 M used by
Murphy [12]. In addition, the growth solution contained a
relatively high concentration of silver nitrate, twice the
concentration of the Au(III) ions. When the four components were
mixed together at 35.degree. C. the gold ions were reduced to the
colorless Au(I) state, forming a [AuX.sub.2].sup.--CTA.sup.+
complex (X.dbd.Cl, Br) but further reduction to the metallic state
required the addition of catalytic metal seed particles. Similarly,
the silver ions formed an AgBr-CTAB complex.
As an alternative, a small amount of sodium borohydride dissolved
in water (e.g., 0.001-0.0001%) was added to the precursor solution
in order to initiate metal reduction in this solution. The
borohydride amount was enough to reduce up to 0.02% of the metal
ions to form small metallic seed particles which catalyzed the
reduction of the rest of the metal ions by the ascorbate.
Immediately after the borohydride addition, the solution was
deposited as a thin film, .about.100 .mu.m thick, on the substrate
of choice that was kept at .about.35.degree. C. and a relative
humidity of .about.50% for drying. The viscosity of the deposited
solution at 35.degree. C. was .about.2 cP for the 0.25 M CTAB
solution and .about.100 cP for the 0.6 M CTAB solution.
FIGS. 5A to 5D display the results of drying the thin growth
solution films on transmission electron microscopy (TEM) carbon
coated grids for samples prepared with two CTAB concentrations:
0.25M and 0.6M. FIGS. 5A to 5B show TEM obtained from 0.25M CTAB
solution. It may be noted that a highly uniform nanowire coating
appeared across the 3 mm diameter grid for the 0.25 M CTAB sample.
Most of the nanowires appeared in wavy bundles with characteristic
bundle size of .about.20 wires, in the case of the 0.25 M CTAB
sample and thicker nanowire domains for the 0.6 M CTAB sample (FIG.
5C). The high magnification image (FIG. 5D) provides more
quantitative information about the structure of the nanowire
bundles; Average nanowire diameters are in the range 2-2.5 nm, and
inter-wire spacing is .about.2.5 nm, which is significantly smaller
than the 3.9 nm estimated for a CTAB bilayer covering thicker gold
nanorods [13]. This difference may be due to a larger radius of
curvature around the ultra-thin nanowires of the invention, which
would lead to a different bilayer packing. Thus, it appears that
the metal was deposited at locally ordered surfactant mesostructure
domains that were previously found to have liquid-crystalline
characteristics, probably close to a reverse hexagonal phase. The
nanowire bundle density and morphology varied with deposited
solution thickness, drying temperature and drying rate (by control
of relative humidity). One of the important parameters was the
initial surfactant concentration; when it was increased to about
0.6 M the liquid crystalline domains were thicker than those formed
at lower concentrations (FIG. 5C), but also with a larger number of
spherical particles that were apparently formed out of the tubular
mesostructures. In the case of the higher CTAB concentration the
formed metal mesostructures bear a closer resemblance to the oxide
based mesoporous materials.
A closer inspection of a sample with high surfactant concentration
(0.6 M) and relatively low silver concentration (4 mM, relative to
the usual 6 mM) revealed regions with discontinuous, segmented
nanowires (inset of FIG. 5C) with typical segment size and
separations of the order of few nm up to .about.30 nm. Accordingly
and without wishing to be bound by theory the nanowire formation
process began in a large number of small metal clusters triggered
by the borohydride addition. These small metal particles were
apparently caught within the surfactant template structure as the
film became progressively more concentrated on drying. While
drying, additional metal atoms deposited on the seeds through
catalytic reduction of the metal ions by ascorbate ions. It has
been previously shown for mesostructured silica that regions of the
mesophase ordered parallel to the interface were induced by
proximity to the interface, as also appears to be the case in the
present invention.
The processes of the invention may be performed using various
different substrates such as silicon, quartz and polyethylene
terphtalate (PET). FIG. 6 displays a scanning electron microscope
(SEM) image of the film as disclosed herein above deposited a
silicon substrate after gentle washing with 70%/30% ethanol/water
solution. In this case it was not possible to resolve individual
nanowires and only whole bundles of the CTAB coated nanowires were
observable.
Conductance measurements of the nanowire films were performed on
various length scales. For example, FIGS. 7A-7D show SEM images of
nanowire bundle, a typical measurement configuration and
current-voltage curves. FIG. 7A shows the current-voltage
measurement for the SEM image shown in FIG. 7B of nanowire bundle
conductance measurement using clean tungsten nanoprobes in the
Zyvex S 100 system. FIG. 7C shows a typical measurement
configuration with the nanowire film deposited on a pre-patterned
silicon substrate with gold electrodes. FIG. 7D shows
current-voltage curves measured with various inter-electrode
spacings as indicated in the legend. On the smallest scale, sharp
tungsten probes (500 nm in diameter) were used in a Zyvex 8100
nanomanipulator system to probe individual nanowire bundles in
situ, while imaging with the SEM, as shown in FIG. 7 A. In order to
avoid large contact resistance the tungsten probes were chemically
cleaned in KOH solution followed by in-situ oxidation removal
process in the SEM, which resulted in a probe-to-probe resistance
of the order of 10.OMEGA.. In addition, the substrate with the
deposited nanowires was thoroughly washed with 70%/30%
ethanol/water and shortly exposed to low-power oxygen plasma, which
removed part of the nanowire film in addition to the surfactant
coating. The current-voltage curves of the nanowire bundles were
ohmic with typical resistance values of the order of 1
k.OMEGA./.mu.m. Several measurements on isolated nanowire bundles
as the one shown in the inset of FIG. 7A were performed. Assuming
an average bundle of 20 nanowires and a diameter of 2.5 nm, an
estimated nanowire resistivity of the order of 10.sup.-7 .OMEGA.m
was obtained, which is about 4 times the resistivity of bulk gold.
Considering the roughness of the estimate and possible probe-wire
contact resistance, this result is roughly consistent with bulk
gold like nanowire resistivity.
In addition, the films were deposited over Si wafers with a 100 nm
thick oxide layer and gold electrodes patterned on top with
inter-electrode 2-20 .mu.m gaps (FIG. 7B). The bundle resistances
measured over these gaps, together with the bundle densities
apparent in the SEM images, were used to estimate effective sheet
resistances that were in the range of 100-300 .OMEGA./sq. They also
exhibited an ohmic behavior down to 4 K. Rough estimates of
nanowires' width and length, connecting the micro-electrodes
provided wire resistivities which are of the same order as bulk
gold (.about.10.sup.-8 .OMEGA.m). This indicates that at least part
of the nanowires grown within the CTAB meso-structures were formed
at the bottom of the dried CTAB film, forming good electrical
contact with the pre-formed gold electrodes. Optical dark field
microscopy confirmed that the nanowire bundles were located at the
bottom of the .about.5-10 .mu.m thick dried CTAB films.
Furthermore, the nanowire films were deposited on a 1 cm.sup.2
fused silica substrates (also from 0.6 M CTAB), silver paint was
applied in two parallel lines at the edges of the substrate and
sheet resistances of the order of 100 .OMEGA./sq were measured
after mild ethanol washing. In particular, the high flexibility of
the film was demonstrated (FIG. 8B) where only up to 10% increase
in the .about.100 .OMEGA./sq sheet resistance occurred for a film
deposited on a PET substrate which was bended with a curvature
radius of .about.1.5 cm. Upon relaxing the bend in the film the
sheet resistance returned to its exact original value,
demonstrating the high flexibility of the nanowire film. The films
deposited on PET have shown the lowest resistivity results, down to
.about.60 .OMEGA./sq.
FIGS. 8A-8C show visible light transmission curve of a nanowire
film, bending of PET substrate coated with a nano wire film and the
periodic table as observed through the PET film. FIG. 8A shows a
visible light transmission curve of a nanowire film deposited on a
fused silica substrate with a sheet resistance of 200 .OMEGA./sq.
FIG. 8B demonstrates bending of a 2.times.2 cm2 PET substrate
coated with a nanowire film to a curvature radius of .about.1.5 cm
maintains a .about.100 .OMEGA./sq sheet resistance. FIG. 8C
displays the periodic table as observed through the same PET film
which had .about.80-85% optical transmission in the visible range.
The bright stripes are silver paint lines used to estimate the
sheet resistance. The upper right corner is film-free.
The optical extinction of the films was measured using a standard
spectrophotometer. A transmission curve for a film with relatively
high transparency and sheet resistance of 200 .OMEGA./sq is
presented in FIG. 8A. Typical far-field transmission of all samples
was in the range of 80-90%. This extinction contained a large
scattering component which, in the case of photovoltaic devices,
may be collected within the device. Without wishing to be bound by
theory, the varying amounts of residual spherical particles, which
had relatively large diameters relative to the nanowires, may be
responsible for a substantial part of the extinction.
Example 3
As recited above, in some experiments, prior to the addition of the
reducing agent (e.g., sodium ascorbate) to the Au precursor
solution, the silver solution was added to the Au precursor
solution and only then the mild reducing agent e.g., sodium
ascorbate was added. Under such conditions no metal reduction was
induced. Subsequently, low concentration (e.g., 1/100 of that of
the sodium ascorbate or lower) of a stronger reducing agent with
respect to ascorbate was added to the solution. Such stronger
reducing agent should have a reduction potential (E.sup.0) of -0.5
V or more negative. Non-limiting examples are sodium borohydride,
sodium cyanoborohydride and hydrazine. The addition of the strong
reducing agent initiated metal reduction in this solution and
subsequent metal deposition on the substrate.
* * * * *